The present invention relates generally to butterfly valves and more particularly to a butterfly valve having an improved connection between the valve shaft and the valve disc.
Butterfly valves have historically been provided with one of two general types of connections between the actuator shaft and the valve disc. In one type of connection the disc includes a portion of increased thickness running longitudinally down the center of the disc and the actuator shaft is passed completely through the center of the disc and pivoted for rotation at the top and the bottom of the valve. While such connections have been generally successful, they suffer from the disadvantage of being relatively expensive due to increased material and machining costs and also suffer from the disadvantage that a somewhat increased restriction to flow is presented when the valve is in the open position. A second general type of actuator shaft connection has been generally called a stub shaft connection. In stub shaft butterfly valves the disc includes portions of increased thickness at the top and the bottom of the disc only. The actuator shaft is partially inserted into the disc at the top of the disc and a small shaft is inserted into the bottom of the disc. Such stub shafts have been connected to the disc in a variety of manners. In some instances the shafts have been pinned or bolted to the disc. Another approach has been to use a broached type connection such as a square connection or a hexagonal connection. Such connections have been subject to the disadvantages of leaking, providing protuberances into the flow stream, or looseness of connection resulting in a backlash making it difficult to close the valve tightly, providing poor control with automatic actuators, or even permitting shaft blow out. Another type of connection which is known and which tends to overcome some of the aforementioned difficulties is a knurled or upset interference fit between the shaft and the valve disc. In this type of connection the shaft is provided with projections on the end thereof, which when forced into contact with the hole in the top of the disc tend to radially deform the hole thereby resulting in an interference fit. This type of connection has suffered from the disadvantage that where a good connection has been made, high stresses are created in the valve disc, which is usually cast iron, increasing the possibility of failure of the valve disc. These high stress levels are required because the disc hub stresses must create a high compressive frictional force in order to fully transmit the valve shaft torque in addition to preventing backlash and preventing the shaft from blowing out due to normal or abnormal internal pressures. While the disc hub may be made larger to reduce the stresses, this increases the cost of material. Furthermore, in those instances where the interference fit is not tight enough, valves may fail under pressure by forcing the shaft axially out of the hole, or by the interference connection being inadequate to transmit the required torque from the shaft to the disc.